polarized photocathode research collaboration pprc r. prepost – university of wisconsin cornell...
TRANSCRIPT
Polarized Photocathode ResearchCollaboration PPRC
R. Prepost – University of WisconsinCornell ALCW July 13-16 2003
• A. Brachmann• J. Clendenin• E. Garwin• T. Maruyama• D. Luh• S. Harvey• R. Kirby• C. Prescott• R. Prepost
Some Considerations
• Technique is Bandgap Engineering of Strained GaAs.• Polarization will be < 100% - But 90% possible.• Active layer must be < 10% of photon absorption length
to preserve strain and polarization.• Uniform Strain over larger thickness in principle possible
with Superlattice structures• Strained GaAs used at SLAC since 1986 with ~85%
Polarization and ~.2% QE.• R & D has been continuous since 1985.
Outline
• Polarized photoemission• Standard SLC photocathode• Surface charge limit• Charge limit vs. doping• Polarization vs. doping• High gradient doped strained GaAsP• High gradient doped strained superlattice• Atomic-hydrogen cleaning• Summary
Polarized photoemission
• Circularly polarized light excites electron from valence band to conduction band• Electrons drift to surface L < 100 nm to avoid depolarization• Electron emission to vacuum from Negative-Electron-Affinity (NEA) surface
NEA Surface – Cathode “Activation”• Ultra-High-Vacuum < 10-11 Torr• Heat treatment at 600° C• Application of Cesium and NF3
Facilities
• QE and Polarization at 20 kV• QE and Polarization at 120 kV under accelerator condition
Standard SLC Strained GaAs
• 100 nm GaAs grown on GaAsP
• Uniformly doped at 51018 cm-3
• Peak polarization ~80%• QE ~0.2 – 0.3%• Max. charge ~7 1011
e-/270ns
GaAs substrate
GaAs1-xPx x=0.3
GaAs1-xPx x=0 0.3
100 nm GaAs
5
4
3
2
1
0
Far
aday
Cup
Sig
nal (
arb.
uni
ts)
3002001000
Time (ns)
Charge saturation
Beam structure
Surface Charge Limit
• Photon absorption excites electrons to conduction band
• Electrons can be trapped near the surface; electron escape prob. 20%
• Electrostatic potential from trapped electrons raises affinity
• Affinity recovers after electron recombination
• Increasing photon flux counterproductive at extremes
TESLA does not have a charge limit problem.
Charge limit (cont.)
pump probe
Two short pulses
Pro
be s
ign
al
Long pulse
Higher doping solves charge limit problem.
Four samples withdifferent doping level: 51018 cm-3
11019 cm-3
21019 cm-3
51019 cm-3
Phys. Lett. A282, 309 (2001)
But higher doping depolarizes spin.
High-gradient doped strained GaAsPNIM A492, 199 (2002)
80% Polarization and No charge limit
E158 cathode
2.5
2.0
1.5
1.0
0.5
0.0
FA
RC
sig
nal (
arb.
uni
ts)
4003002001000
Time (ns)
But polarization is still 80%.
Actual strain is 80% of design
Strain relaxation
Strained-superlattice
Strained GaAs
GaAsP
Strained GaAs
GaAsP
Strained GaAs
GaAsP 30 A
40 A
GaAs Substrate
GaAs(1-x)Px Graded Layer
GaAs0.64P0.36 Buffer
Active Region
25m
25m
1000 A
SBIR with SVT Associates
• July 2001 SBIR Phase I awarded Very first sample produced 85% polarization• Sep. 2002 SBIR Phase II awarded
• MBE growth MOCVD growth• Be doped Zn doped
“Advanced Strained-Superlattice Photocathodes for Polarized Electron Souces”
SLC photocathode
0 20 40 60 80 100 120 140 160 180
Time (s)
RHEEDSignal GaAs Oscillations
AlAs Oscillations
InAs Oscillations
MBE- In Situ Growth Rate Feedback
Monitoring RHEED image intensity versus timeprovides layer-by-layer growth rate feedback
Growth at monolayer precision
Strained-superlattice band structure
GaAs1-
xPx
GaAs GaAs1-
xPx
GaAs GaAs1-
xPx
CB1
HH1
LH1
1.65 eV
86 meV
1.42 eV
b w
Parameters:barrier layer thickness, 30 Å < b < 100 Å well layer thickness , 30 Å < w < 100 Åphosphorus fraction , 0.3 < x < 0.4No. of periods , active layer ~ 1000 Å
Multiple Quantum Well Simulation
Multiple Quantum Well Simulation
• QE ~ Band Gap• Polarization ~ HH-LH
SplittingEffective Band Gap
HH-LH Splitting
Barrier = 5 nm
x
Polarization and QE
• Peak polarization 85%
• QE ~ 0.8 – 1%
• Wavelength dependence is consistent with the simulation.
80
60
40
20
Po
lariz
atio
n (
%)
820800780760740720700680660
Wavelength (nm)
0.01
0.1
1
QE
(%)
w = 3 nm, b = 3 nm w = 4 nm, b = 4 nm w = 4 nm, b = 3 nm w = 5 nm, b = 3 nm
Rocking Curve (004) scan from SVT-3682
GaAs Bulk
GaAs0.64P0.36
Graded GaAs1-xPx
Additional peaks from
superlattice structure
• Both SVT-3682 and SVT-3984 are superlattice cathodes:
– MBE grown Be-doped (SVT Associates).
– Barrier width: 30Å
– Well width: 30Å
– Phosphorus fraction in GaAsP: 0.36
– Layer number: 16
– Highly-doped surface layer thickness: 50Å
• XRD analysis on SVT-3682– Well Width = Barrier Width = 32Å
– Phosphorus fraction in GaAsP: 0.36
No Charge Limit
10
8
6
4
2
0
Ch
arg
e (
x1011
pe
r p
uls
e)
50403020100
Laser Energy (uJ)
60 nsec pulse
Before Cesiation
After cesiation
4
3
2
1
0
Farc
Sig
nal (
V)
12080400
Time (ns)
775nm, 60ns
11012 e- in 60 ns
QE Anisotropy
-10
-5
0
5
10
QE
Ani
sotro
py (%
)
1801501209060300
Angle (degrees)
-3
-2
-1
0
1
2
3
QE
Aniso
tropy
(%)
1801501209060300
Angle (degrees)
EStrain relaxation
Strained GaAs Strained superlattice
10% 1.5%
E158 again
• Cathode installed in May.
• But it shows a charge limit ~71011 e-/300 ns
• Cannot make NLC train charge but OK for E158.
• What happened?• The 600° C heat-
cleaning is destroying the high gradient doping profile.
Atomic-Hydrogen Cleaning
Ga2O3 comes off at ~600° C.Ga2O comes off at ~450° C.
Ga2O3 + 4H Ga2O + 2H2O
600° C heat-cleaning: QE ~ 11%AHC + 450 ° C heat-cleaning: QE ~15%
Bulk GaAs